The present invention relates to a process for preparing ferromagnetic fine particles for magnetic recording comprising barium ferrite particles suited for high-density magnetic recording, especially perpendicular recording utilizing magnetization perpendicular to the recording medium plane.
Longitudinal recording, a recording system utilizing magnetization parallel to the recording medium in which the acicular magnetic particles such as the particles of .gamma.-Fe.sub.2 O.sub.3, cobalt-coated .gamma.-Fe.sub.2 O.sub.3, iron type metals, CrO.sub.2 or the like are oriented parallel to the recording medium plane and remanent magnetization in that direction is utilized for recording, has been most popularly used for magnetic recording. With this recording system, however, it is difficult to realize desired high-density recording as this system involves the problem that an attempt to attain higher level of recording density tends to invite an increase of demagnetizing field in the recording medium to adversely affect its recording and reproducing performance especially in the short wavelength region. For overcoming such problem, attention has been focused recently on the so-called perpendicular recording system in which, in contrast with said longitudinal recording system, magnetization is effected in the direction perpendicular to the recording medium plane so as to decrease the demagnetizing field to thereby enable high-density recording.
As the recording medium for such perpendicular magnetic recording, there have been proposed those made by the alloy film method using Co-Cr alloy or the like whose application to practical use has been tried since long, and more recently the so-called coated type recording media prepared by coating a base film with a dispersion of hexagonal platelet ferrite particles such as barium ferrite particles in a binder resin. The coated-type recording media, like those for longitudinal recording, can be produced at high production efficiency and with economical advantages and are also excellent in durability, so that the realization of their practical application has been an urgent request.
The magnetic barium ferrite particles used for said perpendicular magnetic recording medium are usually composed of hexagonal platelet particles having an axis of easy magnetization in the direction perpendicular to the platelet plane, and they are required to have a large saturation magnetization and a coercive force (usually 200 to 2,000 Oe) suited for magnetic recording and coordinatable with the characteristics of the magnetic head used for recording and reproduction. As regards the size of said magnetic barium ferrite particles, generally the smaller the particle size, the less becomes the noise and the higher becomes the surface smoothness of the recording medium, hence the more advantageous for improving the S/N ratio. Usually a particle size of not more than 0.3.mu., preferably not more than 0.2.mu. is desired. Said particles are also required to have good dispersibility, orientability and packing density.
Various methods have been known for the production of barium ferrite particles to be used for magnetic recording media. Some typical examples of such methods are described below.